The present invention relates to a radio base station apparatus, pilot transmission thereof and a terminal apparatus, and more particularly to a radio base station apparatus that transmits sub frames that include pilots, the pilot transmission method thereof and a terminal apparatus in a downlink radio transmission zone.
Mobile communication systems, for example, cellular phone systems are developing from third generation to fourth generation systems. As this development takes place, new radio access technologies are implemented and the broader radio frequency band is needed, so it is predicted that the maximum data transmission capacity will greatly increase.
OFDM (Orthogonal Frequency Division Multiplex) is employed in the downlink of the radio access portion of EUTRAN (Evolved UTRAN), for which specifications are developed in 3GPP as a next generation cellular system (refer to 3GPP TR25. 814). FIG. 13 is a drawing abstractly explaining a sub frame sequence in the downlink of the radio access portion of EUTRAN, where five sub frames are shown with the horizontal axis being the frequency (downlink transmission band) and the vertical axis being time. As shown in FIG. 14, each sub frame (or slot) comprises 7 symbols (7 OFDM symbols) for example.
An OFDM signal that is transmitted in a 20 MHz wide radio transmission band comprises 1201 subcarriers. In addition, a 20 MHz wide transmission band is separated into 100 sub bands (or Resource Blocks), and when data is transmitted to a terminal, one or more consecutive or dispersed sub bands in the frequency axes are used. One sub band is presumed to comprise 12 subcarriers. The sub frame length is 0.5 ms, and common pilots are transmitted over all of the system transmission bands. The common downlink pilot signals are mainly used for coherent demodulation (channel estimation and compensation) of control signals and user data, and for measurement of the radio link quality. Common pilots are transmitted periodically for each sub frame.
The common pilot signal can be transmitted using all of the subcarriers of an OFDM symbol, however, in the radio access portion of a EUTRAN downlink, the common pilot signal is transmitted by one in every 6 subcarriers from one transmission antenna as shown in FIG. 14, and is transmitted in 2 OFDM symbols of the 7 OFDM symbols.
FIG. 15 is a diagram of transmitter in an OFDM communication system, where a data modulation unit 1 performs data modulation, for example, of transmission data (user data or control data), and converts the modulated data to a complex baseband signal (symbol) having an in-phase component and quadrature component. A time-division multiplexing unit 2 performs time-division multiplexing of the pilot symbols and data symbols. A serial-to-parallel conversion unit 3 converts input symbols to parallel data of M number of symbols, and outputs M number of subcarrier samples. An IFFT (Inverse Fast Fourier Transform) unit 4 performs IFFT (inverse Fourier transform) processing on subcarrier samples that are input in parallel and combines the results of the IFFT processing on the samples, then outputs the result of the combination as a discrete-time signal (OFDM signal). A guard interval insertion unit 5 inserts a guard interval in the OFDM signal that is input from the IFFT unit, and a transmission unit (TX) 6 performs DA conversion of the OFDM signal (called an OFDM symbol) in which a guard interval has been inserted, then converts the frequency of the OFDM signal from a baseband frequency to a radio band frequency, after which it performs high-frequency amplification and transmits the signal from an antenna 7.
FIG. 16 is a diagram of an OFDM receiving device. A signal that is output from the transmission antenna 7 shown in FIG. 15 passes through a fading channel (propagation path) of radio space and is received by a receiving antenna 8 of the receiving device, after which a receiving circuit (Rx) 9 converts the RF signal that is received by the antenna to a baseband signal, and an AD converter 10 converts that baseband signal to a digital signal and outputs the result. An AFC circuit 11 comprises: a first AFC circuit 11a that uses a pilot signal; a second AFC circuit 11b that uses a synchronization signal; and an AFC signal selection circuit 11c; where this AFC circuit 11 estimates the carrier frequency deviation between the terminal and base station, and adjusts the oscillation frequency of the local oscillator inside the receiving circuit.
A symbol extraction unit 12 detects the start of the OFDM symbols; and together with deleting the guard intervals GI, extracts the OFDM symbols and inputs them to a FFT unit 13. The FFT unit 13 performs FFT processing on each extracted OFDM symbol, and converts them to frequency domain subcarrier samples S0 to SM-1. A pilot extraction unit 14 extracts the pilot symbols from the FFT output, and a channel-estimation circuit 15 performs channel estimation for each subcarrier by calculating the correlation between the pilot symbols received at fixed intervals and a known pilot pattern, after which a channel-compensation circuit (synchronization detection unit) 16 uses the channel estimation value to compensate for channel fluctuation of the data symbols. Through the processing described above, transmission data that is distributed to each subcarrier is demodulated. It is not shown in the figure, however, after the demodulated subcarrier signal has been converted to serial data, the signal is decoded. An optimum sub-band setting unit 17 uses the received pilots to measure the reception state of each sub band (radio link quality, for example, SIR), and decides the most suitable sub band. The example of signal processing by the transmission device and reception device shown in FIGS. 15 and 16 is a simplified example, and in actual devices, more complicated processing is performed in order to improve characteristics.
FIG. 17 is a drawing explaining the first AFC circuit 11a that uses a pilot symbol, where an IFFT unit 11a-1 performs IFFT processing of a replica (known pilot) of the pilot signal that is transmitted from the transmission station, and generates a pilot signal that is continuous over time, a correlation calculation unit 11a-2 calculates the correlation between that pilot signal and the received signal, a peak detection unit 11a-3 detect the peak correlation value, and a phase detection unit 11a-4 uses the real portion R and the imaginary portion I of the peak correlation value to calculate the phase difference θ from the following equation.θ=tan−1(I/R)This value θ occurs due to the frequency deviation, so it controls the oscillation frequency of the local oscillator based on the phase difference. The AFC circuit shown in FIG. 17 is an example.
FIG. 18 is a drawing explaining a synchronization signal, (A) is an example where a synchronization channel SCH (synchronization signal) is repeatedly transmitted two times, and (B) is an example where a synchronization channel SCH (synchronization signal) is repeated at the start of a frame and transmitted.
FIG. 19 is a drawing explaining the second AFC circuit 11b, where a delay unit 11b-1 delays the input signal one symbol or one frame, a correlation calculation unit 11b-2 calculates the correlation of the repeated portion, a peak detection unit 11b-3 detects the peak correlation value, and a phase detection unit 11b-4 uses that peak correlation value to calculate the phase difference θ in the same way as in the case shown in FIG. 17, and based on this phase difference, controls the oscillation frequency of the local oscillator. For example, rough correction of the frequency offset is performed using the synchronization signal, and after that fine correction is performed using the pilot signal.
In addition to being used for adjusting the frequency offset as described above, the synchronization signal is also used for detecting the symbol timing, the frame timing, pilot signal pattern, and the like. [0006]
During the period of one day, the amount of data transmission that includes audio data, or in other words, the data traffic, fluctuates. Especially, late at night, the data traffic becomes very low. It is predicted that, during one day, the ratio of the traffic between when the data traffic is at a peak and when the traffic is low, such as during the night, will become larger as the maximum data transmission capacity increases with the change from a third-generation to fourth-generation system.
When data traffic is low such as during the night, the amount of data transmitted becomes low, or in other words, the amount of radio communication resources that is necessary becomes low, and a condition occurs in which the whole transmission band is not fully used. This condition becomes more notable the broader the transmission bandwidth becomes. In a condition in which the data traffic is low, the number of sub bands that are not used for data transmission increases, so there is no need to transmit control signals and data using unused sub bands, as well as there is no need to transmit pilot signals for the purpose of demodulating those control signals and data using those sub bands.
Moreover, in a condition of low data traffic, it is inefficient to transmit the pilots constantly for measuring the quality of the radio link using all of the sub bands. That is because, only a part of the sub bands are used for data transmission.
From the aspect described above, in a condition of low data traffic, it is inefficient to transmit downlink common pilot signals having large power for each sub frame over the whole transmission band (broad transmission band). That is, in a condition of low data traffic, transmitting common pilots using all of the sub bands for each sub frame is inefficient from the aspect of the power consumption required for transmitting the pilots.